The present invention relates to electrical cables, and more particularly to multiply shielded coaxial cables with very low transfer impedance.
Many electrical cables include shielding to reduce signal loss and intercircuit interference. The importance of such shielding is particularly evident in connection with the transmission of large amounts of information in high-frequency bands as in television applications.
Cable shielding serves both ingressive and egressive functions. Limiting the ingress of radio frequency interference (RFI) reduces the distortion and spurious signals that may be induced by electromagnetic fields originating in the cable environment. Limiting the egress of radio frequency (RF) energy limits energy loss from the signals and the contribution of the cable to RFI afflicting neighboring circuits.
Cable shielding usually comprises metal foils, metal braids or both. The foils or braids provide conductive barriers between the cable core and the cable environment while permitting cable flexing. Gaps in the conductive barrier significantly diminish the effectiveness of the shielding. Therefore, braids, which inherently have gaps, often are combined with foils to reduce the gaps and improve effectiveness of the shielding, the braids being used because of their strength and flexibility permitting repeated flexing without rupture.
Simple metal foils thin enough to allow substantial cable flexing often fail structurally. The predominant mode of failure is transverse, a failure known as tiger striping. Many foils are therefore manufactured as a laminate with a strength-giving member, usually of polyester or polypropylene. The strength-giving member helps to maintain the structural integrity of the foil, but prevents the conductive surface from contacting itself where the shield overlaps itself when wrapped around a cable core. Since the strength-giving member is usually nonconductive, a nonconductive gap or slot remains through the shield, permitting the transmission of RF energy therethrough. This leakage can be reduced by providing metal layers on both sides of the strength-giving member, so that there is metal-to-metal contact in the region of overlap. However, as neither metal layer contacts itself, the slot effect is still present.
The combination of braid and foil is well known to be advantageous because of their complementary advantages. See, for example, Wilkenloh U.S. Pat. No. 4,117,260. In addition to the structural strength advantage obtained by the use of braid, braid is well known for low DC resistance, whereas foil reduces gaps in the shielding. The standard combination has been a foil laminate surrounded by a braided metallic layer. For greater shielding effectiveness, it has been known for some time to go beyond the simple combination of a foil with a braid. The next step was to add another layer of foil outside the braid. A standard of the industry is a cable known as type 9110 as manufactured and sold by Belden Corporation, a subsidiary of Cooper Industries, Inc., the assignee of the present application. The Belden 9110 cable has a double foil laminate inner foil surrounded by a metallic braided layer, in turn surrounded by a double foil laminate.
When it became important to provide even more effective shielding, the obvious next step was to add another braided layer, following the well known practice of using the advantages of a braided layer for more effective shielding. Just such a cable has been made and sold by the Times Wire & Cable Company as Times MI-2245 cable. Such cable employs a foil-braid-foil-braid shield that, as expected, has superior shielding effectiveness, as measured by transfer impedance, as compared to prior shields, including the foil-braid-foil shield of Belden 9110 cable.
Transfer impedance as a measure of shielding effectiveness is explained in Kenneth L. Smith, "RF Leakage Test for CATV Drop Cable Gives Absolute Results," TV Communications, Dec. l, 1978, pp. 114-116. The Smith article explains how transfer impedance may be measured and sets forth the transfer impedance characteristic of the Times 2245 cable.
Although Times 2245 cable has been effective and provided an improved transfer characteristic, it has a number of shortcomings. It is not easy to manufacture. It uses much more metal than the Belden 9110 cable. It is expensive. It is bulky. It is the additional layer of braid that makes the cable more costly and bulky, and most significantly of all makes the cable incompatible with standard cable fittings. Certain fittings have become standard for terminating television cables for coupling the cables to one another and to various pieces of television apparatus. It is a nuisance and an expense to have to use special fittings for the Times 2245 cable. There has, therefore, been a need for a cable that provides shielding as effective as the Times 2245 cable that is compatible with standard fittings.
In accordance with the present invention, the solution is to do away with the outer braid and to put what is known as a shorting fold in one of the foil layers, specifically the outer one. A shorting fold is a fold made in the foil laminate so that when the laminate is wrapped around a cable core, a metal layer touches itself at the edges so as to close the slot otherwise formed by the strength member of the laminate. Such shorting folds per se have been known in shielded cables for some time and have been known to be effective at higher frequencies. Conventional wisdom, however, taught that a braided layer was more effective for providing low transfer impedance at lower frequencies and suggested the addition of alternating layers of braid and foil for providing lower transfer impedance, i.e., the Times 2245 cable. At the time applicants made their invention, there was no information as to the actual transfer impedance of a foil-braid-shorting fold foil combination, nor was there any theoretical basis for determining what its transfer impedance might be. There was no way of knowing in advance that the use of the shorting fold would provide a transfer impedance lower than that of the foil-braid foil-braid combination of the Times 2245 cable. Indeed, when applicants first tested their invention, it was to determine how much less effective it would be in respect to achieving low transfer impedance than the Times 2245 cable and whether its lesser effectiveness would not be so bad as not to be offset by the compatibility of the cable with standard fittings. Surprisingly, the new shielding combination proved to be even more effective than the shielding of the Times 2245 cable, as determined by their respective transfer impedance characteristics.
Thus, it is an important aspect of the present invention to provide a cable with improved shielding which is adapted for use with standard connections. In particular, it is an aspect of the present invention to provide a cable with lower transfer impedance and less bulk than the aforementioned Times 2245 cable.